1. Process the jalapeños, cilantro, and onion in a food processor until finely chopped. Add the quartered tomatoes and process until the mixture is fairly smooth but still has some texture. Do not overprocess.
2. Transfer to a small bowl, add the vinegar and salt, and stir to combine. Taste and adjust the seasonings as desired. The salsa is best served within a few hours of making it, but it will keep in the refrigerator, covered, for 3 days.